The mobile fronthaul network based on the radio-over-fiber (RoF) technology is of great interest now due to its capability of transporting broadband wireless signals with extremely high spectral efficiency and low latency. In this paper, we investigate the impact of multipath interference (MPI) on the performance of the RoF-based mobile fronthaul network implemented by using inexpensive directly modulated lasers (DMLs). For this purpose, we first derive an accurate theoretical expression about the power spectral density of the MPI-induced noise. In this expression, we consider the frequency chirp of the DML without using the quasi-static approximation. We then evaluate the carrier-to-noise ratio limited by MPI as a function of the signal-to-interference ratio (SIR). We experimentally confirm the validity of these theoretical analyses and use them to estimate the required SIR of the RoF-based mobile fronthaul link implemented by using a DML. The results show that the SIR should be >55 dB when the mobile fronthaul network is required to transport 480 20-MHz-bandwidth orthogonal-frequency-division-multiplexed signals formatted in 64-quadrature amplitude modulation as in the fifth-generation mobile communication systems.